A printing apparatus having the function of a printer, copying apparatus, facsimile apparatus, or the like, or a printing apparatus used as an output device for a composite electronic device or workstation including a computer, word processor, or the like prints an image on a printing medium such as a paper sheet or thin plastic plate on the basis of image information (including character information or the like). Such printing apparatuses can be classified by the printing method into an ink-jet type, wire dot type, thermal type, laser beam type, and the like.
Of these printing apparatuses, a printing apparatus of an ink-jet type (ink-jet printing apparatus) prints by discharging ink from a printing means (printhead) onto a printing medium. The ink-jet method is superior to other printing methods because the resolution can be easily increased and the ink-jet printing apparatus achieves high speed, quietness, and low cost. On the other hand, needs for color printing have grown, and many color ink-jet printing apparatuses have been developed. As a printhead constituted by integrating and arraying a plurality of printing elements for higher printing speed, the ink-jet printing apparatus uses a printhead in which ink orifices (nozzles) serving as an ink discharge portion and a plurality of liquid channels are integrated. To cope with color printing, the ink-jet printing apparatus generally comprises a plurality of printheads.
FIG. 1 shows the arrangement of a printer part when the printhead prints on a printing sheet surface. In FIG. 1, reference numerals 101 denote ink cartridges. The ink cartridges 101 are comprised of ink tanks which respectively store four color inks, i.e., black, cyan, magenta, and yellow inks, and a printhead 102 having orifices for discharging these inks. FIG. 2 shows orifices arrayed on the printhead 102 when viewed from the z direction. Reference numerals 201 denote orifices which are arrayed in the printhead 102. The orifices are openings at the ends of nozzles, and ink is discharged from the orifices by driving discharge means arranged in the orifices.
Referring back to FIG. 1, reference numeral 103 denotes a sheet supply roller which rotates in a direction indicated by an arrow in FIG. 1 to supply a printing sheet P in the y direction while holding the printing sheet P together with an auxiliary roller 104; 105, sheet feed rollers which feed a printing sheet and also hold the printing sheet P, similar to the rollers 103 and 104; and 106, a carriage which supports the four ink cartridges and moves them along with printing. When no printing is done, or printhead recovery operation or the like is performed, the carriage 106 stands by at a home position (h) represented by the dotted line in FIG. 1.
Before the start of printing, the carriage 106 at the position (home position) in FIG. 1 moves in the x direction upon reception of a printing start instruction, and printing is executed by a plurality of orifices 201 of the printhead 102. When printing ends up to the end of the sheet surface, the carriage returns to the home position and printing is done in the x direction again.
To print an image or the like, various elements such as color development, tone level, and uniformity are required. Especially for uniformity, variations between nozzles that occur due to the printhead manufacturing process, a change over time, or the like influence the ink discharge amount and discharge direction of each nozzle upon printing. The image quality finally degrades to density nonuniformity of a printed image.
In order to verify the discharge state of ink discharged from the printhead that degrades the image quality, a visual verification test pattern is printed. An example of the visual verification test pattern is a visual verification test pattern containing a pattern of straight lines by the number of ink orifices in which one straight line is printed by one ink orifice in the main scanning direction. This visual verification test pattern is used to verify whether the printing position on a specific straight line shifts, the color becomes faint, or the like. The result is used for determination for executing printhead recovery work.
Concrete examples of the cause of degrading the image quality to density nonuniformity of a printed image will be explained with reference to FIGS. 3A to 3C and 4A to 4C. In FIG. 3A, reference numeral 31 denotes a printhead which is constituted by eight nozzles 32; and 33, ink droplets which are discharged from the nozzles 32. Ink droplets are ideally discharged in the same direction by the same discharge amount, as shown in FIG. 3A. If ink is discharged in this manner, dots in the same size are formed on the sheet surface, as shown in FIG. 3B, and a uniform image free from any density nonuniformity as a whole can be obtained (FIG. 3C).
In practice, nozzles vary, as described above. If printing is done in the above fashion, the size and direction of ink droplets discharged from nozzles vary, as shown in FIG. 4A, and dots as shown in FIG. 4B are formed on the sheet surface. In FIG. 4B, blank portions where an area factor of 100% is not satisfied periodically exist in the main scanning direction of the head. To the contrary, dots excessively overlap each other, or blank stripes are formed, as illustrated at the center of FIG. 4B. A set of dots formed in this manner exhibits a density distribution shown in FIG. 4C in the nozzle array direction. These phenomena are generally sensed as density nonuniformity by the human eye. A stripe formed by variations in sheet supply amount may also stand out.
A method of reducing density nonuniformity is disclosed in Japanese Patent Laid-Open No. 06-143618. This method will be briefly explained with reference to FIGS. 5A to 5C and 6A to 6C. According to this method, as shown in FIGS. 5A to 5C, main scanning of the printhead 31 is performed three times in order to complete the same printing region as that of FIG. 4B (FIG. 5A). A region of four pixels which is half of each printing region is completed by two main scanning operations. In this case, the eight nozzles of the printhead are grouped into two: four upper nozzles and four lower nozzles. A dot printed by one nozzle in one main scanning is obtained by substantially halving predetermined image data in accordance with a predetermined pattern. A dot of the remaining half image data is printed in the second main scanning, completing printing of the region of four pixels. This printing method will be called a multipass printing method.
This printing method halves the influence of each nozzle on a printed image even when a printhead identical to that shown in FIG. 4A is used. A printed image as shown in FIG. 5B is almost free from black and blank stripes. As shown in FIG. 5C, density nonuniformity is greatly reduced in comparison with that in FIG. 4C. In this printing, image data is divided in accordance with a predetermined pattern so as to complement each other in the first and second main scanning operations. The pattern is generally one in which pixels are checkered or staggered one by one in the vertical and horizontal directions, as shown in FIGS. 6A to 6C. In the unit printing region (in this case, four pixels), printing is completed by the first main scanning of printing a checkered pattern and the second main scanning of printing an inversely checkered pattern.
FIGS. 6A, 6B, and 6C show a state in which a predetermined region is printed in the use of checkered and inversely checkered thinning patterns. In the first main scanning, a checkered thinning pattern is printed using four lower nozzles (FIG. 6A). In the second main scanning, the sheet is fed by four pixels (½ of the head length), and an inversely checkered thinning pattern is printed (FIG. 6B). In the third main scanning, the sheet is fed by four pixels (½ of the head length), and a checkered thinning pattern is printed (FIG. 6C). Sheet feed by four pixels and printing of checkered and inversely checkered thinning patterns are alternately performed to complete a printing region of four pixels every main scanning.
As described above, according to the multipass printing method, an image is completed by two different nozzles in the same region, and a high-quality image free from any density nonuniformity can be obtained.
In the use of the above-mentioned visual verification test pattern containing a pattern of straight lines by the number of ink orifices in which one straight line is printed by one ink orifice in the main scanning direction, even variations in discharge amount which do not degrade the quality of a printed image are verified as a discharge error particularly when the multipass printing method is adopted.
To prevent recognition of such variations as a discharge error by the visual verification test pattern, various measures are employed for increasing the printhead precision. However, further reduction of variations in characteristic that do not pose any problem in actual printing results in over-quality. Unnecessarily strict quality management in the manufacturing process increases the printhead cost.
This problem is serious particularly in an ink-jet printing apparatus which adopts the multipass printing method. The same problem occurs even in a printing apparatus using a printing method which does not execute multipass printing, other than the ink-jet method.
Demands have arisen for printing and using for visual verification a visual verification test pattern in which variations in printing characteristic that do not pose any problem in actual printing are inconspicuous, a printing element that degrades the quality of a printed image upon actual printing can be determined, and variations between the printing characteristics of printing elements can be verified at the same level as the use in actual printing.